Daily, seasonal, and latitudinal variations in solar ultraviolet A and B radiation in relation to vitamin D production and risk for skin cancer.

Background: Solar ultraviolet (UV) radiation varies with latitude, time of day, and season. Both spectral UV composition and ambient UV dose lead to different health outcomes at different latitudes. Finding the optimal time for sun exposure, whereby the positive effects of UV exposure (vitamin D) are facilitated and the negative effects (skin cancer, photoimmunosuppression) avoided are the most important consideration in modern skin cancer prevention programs.

Objectives: This paper focuses on the latitude dependency of UVB, UVA, vitamin D production, and skin cancer risk in Caucasians.

Methods: Biologically effective UVB (280-315 nm) and UVA (315-400 nm) doses were calculated using radiative transfer models with appropriate climatologic data for selected locations. Incidences of squamous cell carcinoma (SCC) and cutaneous melanoma (CM) were retrieved from cancer registries and published articles.

Results: Annual doses of UVA radiation decrease much less with increasing latitude than annual doses of UVB. Incidences of CM also decrease less steeply with increasing latitude than incidences of SCC. As SCC is caused mainly by UVB, these observations support the assumption that UVA plays an important role in the development of CM. The variations in UVA (relevant to CM) and UVB (relevant to vitamin D production) over 1 day differ: the UVB : UVA ratio is maximal at noon.

Conclusions: The best way to obtain a given dose of vitamin D with minimal carcinogenic risk is through a non-burning exposure in the middle of the day, rather than in the afternoon or morning.